There have been many proposals for the addition of rosins and resins to polyolefin films to improve stiffness, clarity and heat sealability. For example U.S. Pat. No. 3,361,849 suggests the inclusion of rosins and their hydrogenated derivatives. U.S. Pat. No. 3,666,836 and United Kingdon Patent 1 318 137 suggest the inclusion of hydrogenated petroleum resin derivatives and United Kingdon Patent 1231861 suggests that such materials may be useful in twist wrapping.
While in some applications these products have been acceptable they have not proved successful in twist wrapping and this is believed to be due to the difficulties in incorporating relatively large amounts of the resin or rosin derivatives in the polyolefin. As is recognized in United Kingdom Patent 1245250 melt blending is difficult due primarily to the difference in softening/melting temperatures of the polyolefin and the resin or rosin and the difference in their rheological properties in the melt. The process of U.K. Patent 1245250 is both complex and not suited to the production of oriented films.